Where to set script for value already exists

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ASP.NET

Hi ,
I had done to avoid duplicate records while entering in sql query itself now its working fine but not showing any message like value Exists already.so here i entered my c# code

case "N":
               {
                   int userid = Convert.ToInt32(Session["sessionid"].ToString());
                   object[,] objsave = new object[18, 2]
              {
            {"@date_e",convdate },
           {"@date_a",txtDatea.Text.ToString() },
           {"@ContactID",txtcontactid.Text},
           {"@subscriptionTypecode",Convert.ToInt32(ddlsubscriptiontype.SelectedValue)},
           {"@eventid",ddlevent.SelectedValue},
           {"@contacttypecode",ddlparticipated.SelectedValue},
           {"@Donationtypecode",ddldonation.SelectedValue},
           {"@Donationworth",txtdonworth.Text},
           {"@PaymentMode",ddlpaymentmode.SelectedValue},
           {"@PaymentReference",txtpayreference.Text},
           {"@PaymentDate_e",convdtpaydate},
           {"@PaymentDate_a",txtpayDatea.Text.ToString()},
           {"@bankname_e", ddlbankname.SelectedValue},
           {"@bankname_a",txtbanknamea.Text},
           {"@remarks_e",txtremarkse.Text},
		   {"@remarks_a",txtremarksa.Text },
           {"@donatedas",RBLdonatedas.SelectedValue},
           {"@userid",userid}
           
      };
                   ds = DBTask.ExecuteQuery_SP("sp_inserteventparticipationentry", objsave);

                   break;

please suggest me where to do that script